What do you do if you can't use soakaways?
If the site has contamination issues, low permeability in the sub soil (identified by percolation testing) or has a high bedrock/groundwater table then soakaways will not be suitable. Other methodologies available convey the surface water from the roofs and highways into a centralised storage area such as an open pond or an underground tank, with a discharge into the nearest water course.
The discharge rates should be calculated using the pre-development runoff rates (providing that long term storage is provided elsewhere) or the standard 2 l/s/ha, which does include a provision for long term storage.
The existing greenfield (if the site is grassed or has been demolished for a period greater than 5 years) or brownfield runoff volumes need to be calculated again undertaken by our Micro Drainage hydraulic modelling software.
The required storage volume is then calculated using the design return period + climate change over the development site. It is important to get this storage volume correct as over estimating can be expensive to the developer in terms of land grab (ponds) and capital outlay (tanks). Conversely under-estimating can hold up planning applications and can cause costly alterations to any housing layout.
Case Study 1
We proved this during a project with SUDStainability where a layout had already been produced for 16,745m2 of business use, a care home and 450 residential houses in Swindon. The public open space provision was set out at the top of the hill as it was envisaged by the architect for people to take advantage of the fine views in the green park areas; this meant that the housing was located at the bottom of the hill meaning expensive underground drainage tanks would be required. We helped to re-design the housing layout in conjunction with the developer and a landscape architect to move the housing to the top of the site and left the public open space at the bottom of the site, where we designed swales and ponds to store the surface water, which are much cheaper to construct and maintain but crucially provided the required number of treatment trains for appropriate cleaning of the surface water. Tree pits, permeable paving, rain gardens, swales and a combination of dry and wet detention ponds were used in this first-class design.
Case Study 2
We were appointed to provide the necessary surface water calculations for a 30 house development nr Chippenham. We inherited a design where the surface water was draining to an attenuation pond which was located in Flood Zone 3. This solution would have required the provision of a significant amount of flood storage compensation together with the possibility of creating potential problems associated with a surcharged attenuation device and was discounted. We provided a design which involved relaying pipework in the road (in situ) to a new pond located in Flood Zone 1 (low risk). This meant that there was no requirement for providing flood storage compensation consequently unlocking more of the site for development. This obtained approval from Wiltshire Council, Wessex Water and the Environment Agency.